The present invention relates to a reduction gear.
Japanese Patent Application Publication 2005-29117 discloses a conventional reduction gear that includes a pinion spur gear connected to a motor shaft and a large spur gear meshing with the pinion gear. The large spur gear includes a shaft portion, a flange portion and a gear portion. A sun gear is integrally formed with the shaft portion. The sun gear meshes with a plurality of planetary gears, and each planetary gear meshes with a ring gear.
In the above-described reduction gear, the shaft portion and flange portion of the large spur gear are formed integrally. This configuration makes the shape of the large spur gear complex and the manufacturing of the large spur gear difficult, with the result that the production cost may increase. Therefore, it is desirable to form the shaft portion of the large spur gear with which the sun gear is formed integrally and the flange portion having a gear portion separately.
In assembling a shaft having a gear portion and a ring gear that is provided separately from the shaft, the shaft and the ring gear may be fixed together by press-fit, shrink fit, expansion fit or spline fit. With such press-fit, shrink fit and expansion fit, when large torque is transmitted from the ring gear to the shaft, relative rotation may occur between the shaft and the ring gear due to slipping of the ring gear on the shaft. Although the spline fit may prevent the ring gear from slipping relative to the shaft, the shaft and the ring gear are likely to be dislocated relative to each other in the axial direction, so that precise positioning of the shaft and the ring gear in the axial direction becomes difficult.
The present invention, which has been made in light of the above problems, is directed to providing a reduction gear that prevents the relative rotation of the shaft and a ring gear and permits precise positioning of the shaft and the ring gear in the axial direction thereof.